Katsuhiko -- good morning from the UK
I have 2 QRP+ transceivers. One works well the other was purchased to provide spare parts -- which inevitably become 'Unobtanium' over time!
I've repaired a couple of QRP+'s for friends here in the UK. In each case it was the SBL-1 Mixer that had failed.
I did quite a bit of research into why these mixers appear to fail 'spontaneously' and I concluded that it was static electric charge on the antennas so beware when using your 'new' QRP+
Long wires appear to be the worst for this type of failure -- the longer the wire, the more static charge can build up.
I suggest as a precaution you disconnect -- or better still, 'earth' your QRP+ antenna connection when you are not using the rig.
I have a 6-way antenna switch between my QRP rigs and my antennas at home and it enables me to 'earth' any or all the rigs if needed.
There is a small modification you can do to the QRP+ to reduce the likelihood of this failure mode. There is information on the Web about this.
Enjoy your QRP+?
I use mine every week on CW and make many contacts from the UK.
73
Bruce G4ABX
g4abx